The present invention relates to noise reduction devices and more particularly to a muffler for the combustion air intake of a burner tube of an industrial burner.
At the present time there are various types of burners used for industrial applications. For example, one type of industrial burner utilizes propane as its fuel. Such burners may include an elongated combustion chamber (burner tube) having a combustion air intake at one of its ends and an exhaust means, such as an exhaust stack, connected to its opposite end. The gaseous fuel is injected and burns within the elongated chamber.
One use of that type of burner is as a "propane vaporizer." A propane vaporizer may be used to add heat to liquid propane to convert the liquid propane into a gas which is then burned to heat drying ovens or for other purposes. One type of propane vaporizer comprises an elongated U-shaped burner tube, which is its combustion chamber and which is surrounded by water within an enclosing tank. One or more coils are connected to the liquid propane storage tanks and connected through the outer wall of the enclosing tank and through the water. Propane is burned along with air in the combustion chamber and heats the water to convert the liquid propane into gaseous form. For example, the burner tube combustion chamber may be 13 inches in diameter and 20 feet in length.
In the prior art, an intake air orifice of the burner tube is covered by a wind screen at one end of the combustion chamber and its opposite end is connected to an exhaust stack. The wind screen (wind shield) is a metal plate having louvres to prevent gusts of wind from directly entering the combustion chamber and is vertical relative to the ground. Generally the combustion occurs with a low-frequency rumbling noise which is hardly noticeable except to workers in its immediate vicinity. However, under certain wind conditions a low-frequency high-amplitude noise may be produced. The noise and vibration may be so loud and objectionable that persons 100 feet away find it difficult to work or to concentrate.
The inventor has solved that noise problem by his invention of the silencer device of the present invention, which tends to keep the air pressure at the burner air inlet orifice relatively steady. The silencer device of the present invention accomplishes such relatively steady pressure by utilizing a front air port leading around a core member and a rear circumferential air port. When wind blows at an angle toward the front end of the silencer device, it will raise the air pressure at its front end air intake port and lower the pressure at its rear air intake port. Since air is taken through both air intake ports, the pressure within the silencer device at the air intake orifice of the burner tube will stay relatively steady.
Upon an analysis of the problem, it was found by the present inventor that the loud noise was produced in the prior art burner only when the wind blew in certain directions relative to its air intake orifice. The loud noise was not produced when the wind was directed more or less directly into, or directly away from, the wind screen. On the other hand, when the wind was more or less across the wind screen, for example, about 30.degree. to 80.degree. relative to an imaginary axis running through the burner tube and through the wind screen, the loud noise may occur. It was hypothesized that the loud noise was due to a standing noise wave which was started within the burner tube by the wind. The standing wave, as it reinforced itself over and over again by reflections from the two opposite ends of the burner tube, became so loud as to pose a noise pollution problem. The reflecting surface at one end of the burner tube was the wind screen and the reflecting surface at the other end was the curved portion of the U-shaped burner tube.
There have been, in the patent literature, various proposals for absorbing or reducing sound. For example, in U.S. Pat. No. 3,692,140 to C. D. Smith, a sound absorber is shown which includes a perforated metal sheet which is backed by a sound absorbent material. The sound suppressor of the Smith patent was used as the muffler on a gas turbine. The device illustrated in FIG. 5 of the Smith patent includes an interior sound absorber which is described and being circular and concentric. In U.S. Pat. No. 2,964,121 to J. S. Zink, Jr., a muffler for a gaseous fuel aspirator is shown which includes a skirt portion having a perforated metal liner backed by a sound absorbing material. As illustrated in its FIG. 2, the air direction 56 is around the flared end portion 24 of the inspirating device.
In U.S. Pat. No. 3,819,319, and similarly in U.S. Pat. No. 3,840,326, both to Schreter, a noise attenuator for a turbocompressor is placed in air flow communication with the combustion chamber air inlet. The Schreter U.S. Pat. No. 3,819,319 states that the low-frequency noise is the most difficult to attenuate and that a Helmholtz chamber has been incorporated into the combustion chamber to absorb and to selectively attenuate the noise of the low-frequency range (col. 12, lines 66-68). In addition, another acoustic attenuator is placed in "air flow communication chamber air inlet" (col. 2, lines 13-14). The second acoustic attenuator has partitions defining a plurality of passageways. Each of the partitions is hollow and made from spaced metal sheets and "joined at the rear by riveting, welding or the like to form a tapered trailing edge" (col. 14, lines 13-15). A sound absorbing material fills each of the enclosed partitions. In U.S. Pat. Nos. 2,580,655 (Chipley), 2,801,518 (Wosika) and 3,748,085 (Peopsel), silencers are used in connection with an air inlet and include a perforated wall which is backed by a sound absorbing material.